Hydraulic drive systems that employ a reciprocating piston can be employed to provide reciprocating actuation for a wide variety of applications. With such drive systems, the hydraulic piston travels within a cylinder between two opposite cylinder heads. To move the hydraulic piston in one direction hydraulic fluid is delivered from a hydraulic pump to a first chamber that is associated with one side of the hydraulic piston while hydraulic fluid is drained from a second chamber that is associated with the other side of the hydraulic piston. To reverse the direction that the hydraulic piston is traveling, the hydraulic fluid flow direction is reversed so that hydraulic fluid is drained from the first chamber and hydraulic fluid from the hydraulic pump is delivered to the second chamber. A piston rod is attached to the hydraulic piston at one end and to the machine to be driven at the other end, and in this way, the hydraulic drive system can provide reciprocating movement to the machine to which it is operatively connected. For many applications, the efficiency and performance of the machine depends upon the hydraulic piston traveling a consistent distance in each actuation stroke. An example of a machine with such a requirement is a reciprocating piston pump because the hydraulic drive system drives a reciprocating pump piston and the efficiency and performance of such a pump relies upon a consistent pump piston stroke that reduces dead volume at the end of each power stroke. Accordingly, there is a need for hydraulic drive systems with hydraulic piston actuators that can provide piston strokes of consistent length.
Hydraulic cylinders can be designed with a piston stop that provides a physical limit for stopping the piston at the cylinder head or at a shoulder near the cylinder head. However, to reduce noise, wear and/or to prevent more severe damage to the piston stop, a means of detecting when the piston has reached the piston stop is needed so that the hydraulic fluid flow can be reversed to switch the direction of piston movement.
Conventional hydraulic actuators are known to employ position sensors, such as magnetic switches, for detecting when the actuator piston has reached the piston stop that defines the end of a piston stroke. When the position sensor detects the hydraulic piston the sensor sends a signal to a controller and the controller commands a flow-switching valve to reverse the hydraulic fluid flow so that the hydraulic piston reverses direction. A disadvantage of such conventional arrangements is that it requires at least one position sensor that adds to the cost of the system. With conventional arrangements such as this it can also be difficult to adjust the timing for reversing hydraulic fluid flow responsive to changes in hydraulic fluid flow rate, which affects piston velocity. In addition, conventional systems like this often require a pressure relief valve to prevent over-pressurization of the hydraulic system, for example, if there is a malfunction of the position sensor.
U.S. Patent Application Publication No. US2003/0079603 A1 (application Ser. No. 10/317,311), entitled “System For Controlling Hydraulic Actuator” discloses a method whereby a fluid flow sensor is employed to measure the hydraulic fluid flow traveling into and out of the hydraulic actuator cylinder. With the known dimensions of the hydraulic actuator cylinder it is possible to measure the hydraulic fluid flow rate and calculate the position of the piston. With this information it is also possible to calculate the velocity of the piston and the direction of movement. However, fluid flow sensors are relatively expensive, and in a hydraulic system that employs a plurality of actuators, a fluid flow sensor is needed for each actuator. Also, the precision of such a system is dependent very much upon the accuracy of the fluid flow sensor.
U.S. Pat. No. 4,213,298 (the '298 patent) discloses a self-reversing hydraulic control system that uses only mechanical devices for reversing hydraulic fluid flow. A special flow-sensing valve senses changes in hydraulic fluid pressure that are indicative of when the hydraulic piston has come against a physical limit. The flow-sensing valve diverts hydraulic fluid to flow to valves that hydraulically actuate a hydraulic fluid flow-switching device that reverses hydraulic fluid flow to reverse the direction of movement of the hydraulic piston. In the '298 patent, the inventors claim that their invention is particularly advantageous for marine applications where electrical components can be adversely affected by long term exposure to salt air and salt water. Another feature noted by the '298 patent is that the flow-sensing valve also operates to change the hydraulic piston direction when the hydraulic piston is blocked by an obstacle before completing a piston stroke. However, a disadvantage of this solution is that it requires more mechanical components, which require more space, add more weight to the system, and add to manufacturing and maintenance costs.
Canadian Patent No. 1,247,984 discloses a valve for use with hydraulic ram assemblies. The valve operates to inhibit fluid by-pass through the piston when the piston changes direction as a result of either shock loading or intentionally high operational loading. According to the '984 patent, the sudden or abrupt change in direction of the piston can be responsible for reverse flow or by-pass of fluid from the non-pressure side of the piston to the pressure side, before and/or after impact or contact with the pushrod and cylinder end. An objective of the valve disclosed by the '984 patent is to alleviate fluid leakage or by-pass through the piston by providing a valve that comprise a chamber that is held closed to the low pressure side and that can open to the pressure side responsive to a pressure pulse caused by shock loading. The disclosed valve comprises two valve members that are each biased in respected closed positions by a spring. By allowing hydraulic fluid to flow into the valve chamber, the valve acts as a means for relieving hydraulic pressure and reducing the magnitude of the pressure pulses in the high-pressure side. Hydraulic fluid can flow through the valve when the piston is at the end of a piston stroke. A disadvantage of the valve disclosed by the '984 patent is the number of parts. In addition, the '984 patent does disclose a method of controlling the timing for switching piston direction.
Accordingly, there is a need for a simpler, less expensive hydraulic system and method of effectively controlling the reversal of piston movement at the end of each piston stroke, without the use of position sensors, flow rate sensors, or special flow-sensing valves.